Unforgettable 9/11 images

Smoke pours from the twin towers of the World Trade Center after they were hit by two hijacked airliners in a terrorist attack, Sept. 11, 2001, in New York. Nearly 3,000 people were killed after two hijacked planes crashed into the World Trade Center, one into the Pentagon in Arlington, Va., and one in Shanksville, Pa.

Credit: Robert Giroux

Smoke rises from the New York skyline on Sept. 11, 2001, after two hijacked planes crashed into the landmark World Trade Center. Nearly 3,000 people were killed after two hijacked planes crashed into the World Trade Center, one into the Pentagon in Arlington, Va., and one in Shanksville, Pa.

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Smoke billows from one of the World Trade Center's twin towers after it was struck by a commercial airliner on Sept. 11, 2001, in New York City.

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A helicopter flies above the World Trade Center after the landmark twin towers were hit by two hijacked commercial aircraft on Sept. 11, 2001, in New York.

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Smoke billows out of the burning World Trade Center towers before their collapse, Sept. 11, 2001 in New York.

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The twin towers of the World Trade Center billow smoke after hijacked airliners crashed into them early Sept. 11, 2001.

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Smoke pours from the World Trade Center after being hit by two planes on Sept. 11, 2001, in New York.

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Pedestrians on the waterfront in Brooklyn, New York, look across the East River to the burning World Trade Center towers, Sept. 11, 2001.

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The World Trade Center burns after two planes hit it on Sept. 11, 2001 in New York.

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This is a view of the burning World Trade Center towers from Park Place Street and West Broadway Avenue on Sept. 11, 2001.

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The World Trade Center burning after two airliners crashed into the buildings in New York, Sept. 11, 2001.

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Policemen and firemen run away from the huge dust cloud caused as the World Trade Center's Tower One collapses after terrorists crashed two hijacked planes into the twin towers, Sept. 11, 2001, in New York City.

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People evacuate the area around the World Trade Center after it was hit by two planes on Sept. 11, 2001, in New York.

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A truck sits in the rubble in lower Manhattan, Sept. 11, 2001, in New York after two planes flew into the World Trade Center twin towers.

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Photo dated Sept. 11, 2001, shows firefighters making their way through the rubble of the World Trade Center in New York after two hijacked planes flew into the landmark skyscrapers.

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This photo taken on Sept. 11, 2001, shows Marcy Borders covered in dust as she takes refuge in an office building after one of the World Trade Center towers collapsed in New York.

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This photo taken on Sept. 11, 2001, shows mannequins on the floor of the Brooks Brothers' shop in New York near the World Trade Center after the twin towers collapsed.

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Firemen walk amid the smouldering rubble of the World Trade Center following a terrorist attack on Sept. 11, 2001, in New York.

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An unidentified New York City firefighter walks away from Ground Zero after the collapse of the Twin Towers, Sept. 11, 2001, in New York.

Credit: Anthony Correia

A police scooter sits in the rubble in lower Manhattan on Sept. 11, 2001, in New York after two planes flew into the World Trade Center twin towers.

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Photo dated on Sept. 11, 2001, shows firemen make their way through rubble and debris after the collapse of one of the World Trade Center Towers in New York.

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A building stands in ruins at the World Trade Center after two hijacked planes crashed into the Twin Towers on Sept. 11, 2001, in New York.

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Dust swirls around south Manhattan moments after a tower of the World Trade Center collapsed on Sept. 11, 2001, in New York City.

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Smoke cover a destroyed building at the World Trade Center after two hijacked planes crashed into the Twin Towers, Sept. 11, 2001, in New York.

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People walk in the street in the area where the World Trade Center buildings collapsed on Sept. 11, 2001, after two airplanes slammed into the twin towers in a suspected terrorist attack.

Credit: Mario Tama

A firefighter breaks down after the World Trade Center buildings collapsed Sept. 11, 2001, after two hijacked airplanes slammed into the twin towers in a terrorist attack.

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A survivor sits outside the World Trade Center after two planes hit the building Sept. 11, 2001, in New York.

Credit: Angela Jimenez

People evacuate the area around the World Trade Center after it was hit by two planes Sept. 11, 2001, in New York.

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The rubble of the World Trade Center smoulders following a terrorist attack, Sept. 11, 2001, in New York.

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Smoke comes out from the west wing of the Pentagon building, on Sept. 11, 2001, in Arlington, Va., after a plane crashed into the building and set off a huge explosion.

Credit: Alex Wong

President George W. Bush being informed by his chief of staff Andrew Card of the attacks on the World Trade Center in New York during an early morning school reading event on Sept. 11, 2001, in Sarasota, Fla.